For years I never used additives, but recently I used a bottle of Regane in 3 vehicles with MMO following in the gas, and soon I'm going to add some MMO in my daily driver RAV oil, since it looks like some varnish is building up. It's not bad, so I'll add a light dose for 500 miles then do an oil change, repeat as I think necessary.
I'm also running MMO now in the gas for two motorcycles and OPE.